1. Technical Field
The present invention relates to a technique for recording information using light.
2. Background Art
Parts of technical terms in following descriptions are those used in Blu-ray Disc (BD). These terms are sometimes referred to as other words in systems other than BD. However, since those skilled in the art could readily understand such words, technical terms in BD system will be used below.
Storage capacities of optical discs have been expanded mainly by reducing wavelength of light sources, by increasing numerical apertures (NA) of objective lenses, and by increasing the number of recording layers per one disc. In dual layer BD systems, recording capacity of 50 GB have been achieved using blue semiconductor lasers and high NA objective lenses with NA of 0.85. However, reducing wavelengths of recording/reproducing light or increasing NA of objective lenses have almost reached their limits.
Under such circumstances, a possible method for further increasing recording capacity of optical discs may be to increase linear recording density by simply shrinking channel bit length, thereby increasing surface recording density. By increasing the number of recording layers up to 3-4 layers in addition to above, BDXL with recording capacity of more than 100 GB has been achieved. However, this method intensifies inter symbol interferences, thus reducing resolution of short marks or spaces. In BDXL, resolution of the minimum marks and spaces is 0. By further shrinking channel bit length, resolution of the second minimum marks and spaces will be 0. Those skilled in the art readily understand that decoding process of PRML method will not work under such configurations. In other words, this method has a limit in significantly improving recording density.
Another method for increasing recording capacity of optical discs is code modulation. A type of code modulation has already been used in BD or the like. It is expected that code modulation will achieve several advantageous effects. Improving linear recording density is one of the most expected effects among them. Run-length limit code is known as one of methods used for that objective.
In optical discs, spot diameter of light used for reproduction is much larger than physical resolution of recording medium. Therefore, if binary data to be recorded (referred to as user data in this document) is recorded in association with existence/non-existence of recording mark, the margin between recorded bits may be smaller than the diameter of light spot. This drastically makes it difficult to recognize codes due to inter symbol interferences between adjacent bits. As a result, the resolution of recording medium cannot be utilized efficiently.
On the other hand, in run-length limit code, user data is recorded after converting it into a code sequence described by length of marks and spaces. Even if the unit length of marks and spaces (channel bit length) is smaller than the light spot diameter, it is possible to identify the lengths of marks and spaces in the temporal axis during reproduction. Note that the minimum marks and spaces have a length longer than 2 channel bits so that they can be reproduced with sufficient resolutions. In this way, it is possible to achieve a higher linear recording density than that of optical systems having the same special resolution.
When recording information using run-length limit codes, it is principally appropriate to discuss lengths of both recorded marks and spaces. However, for the sake of simplifying descriptions, only marks will be discussed when handling recorded marks and spaces in the same way as long as no confusion will be incurred. For example, “resolution of the minimum mark” means “resolution of the minimum mark and space”.
Two types of run-length codes are known. The first one is fixed length code on the basis of enumeration. The second one is variable length code. The run-length limit code used in BD, which is a current representative optical disc, is a variable length code with a minimum run length of 1. It achieves a linear recording density that is four-thirds times larger than that of without code modulation.
Non-Patent Document 1 listed below describes an algorithm to generate, while satisfying the minimum run-length limitation, fixed-length channel bit words corresponding to user bit words. Since those skilled in the art will readily understand the algorithm, its details will not be described in this document. With this algorithm, it is possible to mathematically calculate fixed-length channel bit words from given fixed-length user bit words (channel bit word generation). Similarly, from channel bit words, it is possible to calculate corresponding user data words using simple mathematic calculations (channel bit word demodulation).